As of Space 11.4 our understanding is that it should be the two initial appliances installed on the same network subnet, (can be a mix of physical and virtual) and then one standard license with enough of a device count to cover the number of devices being managed. You can add up to 4 additional appliances on different networks into the Space Fabric but the third appliance onwards only runs the application logic.
The concept for HA in JunOS Space is the JunOS Space Fabric:
So the BOM for your HA configuration should be two appliances (with hardware support contracts if they are physical appliances), one standard license with the device count you are looking for, and the related software support contract to match.
The advanced licensing may work the same way - haven't looked at this for a while.
